1. Field of the Invention
The present invention relates to a directional drilling tool for drilling boreholes into the earth. More specifically, the present invention relates to an apparatus comprising a number of movably mounted cutting elements which are movable between first radially retracted positions and radially extended positions for cutting. A rotary valve is provided for synchronizing the movement of the cutting; and, control of the directional drilling system is affected by synchronized movement of the cutting elements from an inner to an outer radial position in accordance with the angular position of the drill bit.
2. Description of the Related Art
Drilling of bore holes is conducted for the exploration and production of hydrocarbon fuels, for example in gas and oil exploration and production. The term “directional drilling” is used to describe the process of drilling a bore hole which is directed, for example, towards a target or away from an area where the drilling conditions are difficult. A directional drilling tool generally sits behind a drill bit and forward of measurement tools. The complete system of bit, directional and measurement tools is called the bottom hole assembly, or “BHA”. Currently, there are two main types of directional drilling tools, namely positive displacement mud motors and rotary steerable directional drilling tools.
Positive displacement mud motors are placed in the bottom hole assembly behind the drill bit and operate in either a “sliding” or “rotating” mode. When in sliding mode the drill string is held stationary at the surface. Fluid is then pumped through the positive displacement motor which is situated above the drill bit and connected to the drill bit by a drive shaft and universal joint. Generally there is a fixed bend in the collar between the bit and motor in order to offset the drill bits axis of rotation with the axis of rotation of the BHA. The drill bit will then tend to head in the direction of the bend. By controlling the angle of the bend relative to the formation being drilled, the drilling direction can be controlled. However, the angle of the bend can only be controlled from the surface and measurements of the bend position, commonly known as tool face angle, are sent to the surface using some form of up-hole communication device. As drilling progresses, the BHA advances forward and the rest of the drill string slides along the well bore, hence the term “sliding”.
In order to control the rate of turn of the well bore being drilled, the drill string is rotated from the surface while the motor is rotating the drill bit. This effectively cancels the effect of bend between the motor and drill bit. The drill bit will thus head straight ahead. This is commonly known as rotating.
This method of directional drilling, alternating between rotating and sliding, is slower than continual rotation of the drill string from the surface due to the torque limitation of mud motors, and hence slow rates of penetration are achieved when operating in the sliding mode.
Directional drilling while continually rotating the drill string offers the following advantages: better hole cleaning; smoother well bores, extended reach drilling and higher rates of penetration. However, these tools are often complex in design and hence are costly to manufacture and operate.
For example, UK patent application No. GB2259316 describes a modulated bias unit for steerable rotary drilling systems. The modulated bias unit comprises one or more pads which press against the side of the formation being drilled to exert a lateral force on the drill bit. By controlling the direction of the force the drill bit can be steered into the required direction. This enables the drill bit to cut across as well as forwards and is commonly known as “push-the-bit”.
Another method involves pointing the bit in the intended drilling direction. For example, International patent application WO0104453 describes a method of deflecting a bit shaft, which runs through the centre of the drilling tool. Deflecting the shaft angles the bit with respect to the remaining parts of the BHA. The bit shaft can be permanently deflected and the position of the deflection controlled, or both the position and magnitude of the deflection can be controlled. These systems typically use a non rotating sleeve which presses against the formation which can be problematic if the hole is drilled slightly over gauge (over size).
“Point-the-bit” drilling can also be performed by contra-rotating a bit shaft in a fixed radius and at a rotation rate equal but opposite to the drill string rotation. For example, International patent application WO9005235 describes such an arrangement. Again this offsets the bit axis of rotation relative to the rest of the BHA and the drill bit will tend to move in the direction of the off-axis offset.
UK patent application No. 0602829.4 describes a directional drilling device for use in drilling boreholes, the device being positionable between a drill bit and associated drill collar of a drill string having a longitudinal drilling axis. The device comprises at least one cutting member movably mounted with respect to a tool body member, the cutting member(s) being movable between a first extended position for engagement with the wall of a bore hole and a second position in which it is retracted from engagement with the wall, and directional control means for synchronizing the movement of the cutting member(s) between the respective extended and retracted positions in accordance with the rotational position of the body member in the bore hole being drilled.
As the moveable cutter, or cutters, are extended and retracted from the rock formation being cut it is possible that over a prolonged period of operation chipping of the cutting faces of the moveable cutter or cutters could occur. Cutter chipping is well known in the art of PDC drill bits and normally occurs when the cutter is removed from the rock formation and then is forced back into the formation during cutting operations. Chipped PDC cutters do not cut efficiently and can lead to undersize holes being drilled and, in extreme cases, result in the drilling operation being terminated prematurely.
Accordingly, there is a need for an improved apparatus and method of controlling the drilling direction of a rotary drill string when drilling boreholes in subsurface formations.